Not applicable.
The present invention relates generally to sensors and more particularly to a method and apparatus for sensing the speed and direction of a moving object and for providing information regarding the environment in which a sensor is disposed.
As is known in the art, magnetic sensing devices which can detect the presence of a ferromagnetic object in the vicinity of the sensing device have been widely used. Such sensing devices typically utilize a magnetic field and employ sensing apparatus that detect changes in the strength of the magnetic field. Magnetic field strength is defined as the magnetomotive force developed by a permanent magnet per the distance in the magnetization direction. As an example, an increase in the strength of a magnetic field, corresponding to a drop in the reluctance of a magnetic circuit, will occur as an object made from a high magnetic permeability material, such as iron, is moved toward the magnet. Magnetic permeability is the ease with which the magnetic lines of force designated as magnetic flux, can pass through a substance magnetized with a given magnetizing, force. Quantitatively, it is expressed as the ratio between the magnetic flux density (the number or lines of magnetic flux per unit area which are perpendicular to the direction of the flux) produced and the magnetic field strength, or magnetizing force. Because the output signal of a magnetic field sensing device is dependent upon the strength of the magnetic field, it is effective in detecting the distance between the sensing device and an object within the magnetic circuit. The range within which the object can be detected is limited by the flux density, as measured in Gauss or teslas.
As is also known, where it is desired to determine the speed or rotational position of a rotating object, such as a disk mounted on a shaft, the object is typically provided with surface features that project toward the sensing device, such as teeth. The proximity of a tooth to the sensing device will increase the strength of the magnetic field. Accordingly, by monitoring the output of the sensing device, the rotational speed of the disk can be determined by correlating the peaks in the sensor""s output with the known number of teeth on the circumference of the disk Likewise, when the teeth are irregularly spaced in a predetermined pattern, the rotational position of the body can be determined by correlating the peak intervals with the known intervals between the teeth on the disk.
One prominent form of such a sensing device is a Hall effect transducer. A Hall effect transducer relies upon a transverse current flow that occurs in the presence of a magnetic field The Hall effect transducer is primarily driven by a direct current (DC) voltage source having electrodes at both ends of the Hall effect transducer, creating a longitudinal current flow through the sensor""s body. In the presence of a magnetic field, a transverse voltage is induced in the transducer that can be detected by a second pair of electrodes transverse to the first pair. The second pair of electrodes can then be connected to a voltmeter to determine the potential created across the surface of the sensor. This transverse voltage increases with a corresponding increase in the magnetic field""s strength.
The Hall effect transducer can also be chopper stabilized, wherein for half of some clock cycle the first pair of electrodes receives the DC voltage source creating the longitudinal current flow through the sensor""s body. For the second half of the clock cycle, the second pair of electrodes receives this energizing DC voltage. This well-known technique reduces the inherent electrical offsets that may be present in the Hall effect transducer.
The Hall effect transducer is most often integrated into a single integrated circuit that contains conditioning circuitry to amplify and otherwise modify the output of the Hall effect transducer. This integrated circuitry is often referred to as a Hall effect sensor.
The Hall effect sensor is mounted within and perpendicular to a magnetic circuit that can include a permanent magnet and an exciter (the object being sensed). The exciter is a high magnetic permeability element having projecting surface features which increase the strength of the magnet""s magnetic field as the distance between the surface of the exciter and the permanent magnet is reduced. Typically, the exciter will be in the form of a series of spaced teeth separated by slots, such as the teeth on a gear. The exciter moves relative to the stationary Hall effect sensor element, and in doing so, changes the reluctance of the magnetic circuit so as to cause the magnetic flux through the Hall effect element to vary in a manner corresponding to the position of the teeth. With the change in magnet flux there occurs the corresponding change in magnetic field strength, which increases the transverse voltage of the Hall effect sensor.
The Hall effect sensor can also detect the proximity of a permanent magnetic material, such as a rotating ring magnet.
With the increasing sophistication of products, magnetic field sensing devices have also become common in products that rely on electronics in their operation, such as automobile control systems. Common examples of automotive applications are the detection of ignition timing from the engine crankshaft and/or camshaft, and the detection of wheel speed for anti-lock braking systems and four-wheel steering systems. For detecting wheel speed, the exciter is typically an exciter wheel mounted inboard from the vehicle""s wheel, the exciter wheel being mechanically connected to the wheel so as to rotate with the wheel. The exciter wheel is provided with a number of teeth which typically extend axially from the perimeter of the exciter wheel to an inboard-mounted magnetic field sensor. As noted before, the exciter wheel is formed of a high magnetic permeability material, such as iron, such that as each tooth rotates toward the sensor device, the strength of the magnetic field increases as a result of a decrease in the magnetic circuit""s reluctance. Subsequently, the magnetic circuit reluctance increases and the strength of the magnetic field decreases as the tooth moves away from the sensing device. In the situation where a Hall effect device is used, there will be a corresponding peak in the device""s potential across the transverse electrodes as each tooth passes near the device.
The sensor""s output is dependent upon the distance between the exciter and the sensing device, known as the air gap. More specifically, as the air gap increases, the maximum output range of the device decreases thus decreasing the resolution of the output and making it more difficult to accurately analyze the device""s output. The output of a Hall effect device is directly proportional to the strength of the magnetic field, and therefore is sensitive to the air gap at low strength magnetic fields.
An integrated circuit magnetic field sensor assembly typically can include a magnet, and one or more of the above-described Hall effect semiconductor sensors, where the magnetic poles are sensed as they move relative to the Hall effect semiconductor sensor. The assembly can also include a permanent magnet mounted proximal to the Hall effect semiconductor sensor, and a ferrous object then moves relative to the Hall effect semiconductor sensor, and this sensor detects the disturbances in the magnetic field created by the passing ferrous object. The magnet provides a magnetic field. The semiconductor sensors are located within the magnetic field and are utilized for sensing the strength of the magnetic field. The magnetic field sensor allows the detection of a ferromagnetic object passing through the magnetic field. The magnetic field sensor is disposed adjacent to the ferromagnetic object and is positioned from the ferromagnetic object so as to reduce the distance between the magnetic field sensor and the ferromagnetic object and still maintain an air gap between the magnetic field sensor and the ferromagnetic object to allow passage of the ferromagnetic object by the magnetic field sensor. The semiconductor sensors may be realized as Hall effect devices which are used to detect edges of the ferromagnetic object such as a gear tooth. From the detection of the edges of a gear tooth, information relating to the speed and direction of the rotating gear can be determined.
While detecting the speed of and direction of rotating devices has proven useful, further information regarding the rotating device is generally not available. It would, therefore, be desirable to provide a method and apparatus which provides information relating to the speed and direction of a rotating device and to further provide information relating to the environment the device is disposed in.
A method and apparatus for providing information from a speed and direction sensor is disclosed. The method and apparatus detect the presence of a ferromagnetic object as it moves past a sensor, or detect the disturbances in a magnetic field created by a ferrous object as it moves past a sensor having a permanent magnet in close proximity. The sensor determines speed and direction information regarding the ferromagnetic object, and further provides information relating to the environment surrounding the sensor or object, such as the status of an air gap between the sensor and the moving object, and the temperature of the environment in which the sensor or object is disposed.
In accordance with the present invention, a method of providing information from a speed and direction sensor includes generating at an output of the sensor a data string having a plurality of pulses, wherein a first logic data bit is represented by a first pulse having a first width and a second logic data bit is represented by a second pulse having a second pulse width wherein a first one of the first and second pulse widths is a multiple of a second one of the first and second pulse widths.
With this particular arrangement, a sensor which provides a data string in accordance with a predetermined protocol is provided. In one embodiment, the method includes the step of forming one or more data words using at least two pulses, wherein the data word conveys a characteristic of at least one of a target and an environment in which the target is disposed. By utilizing a protocol in which data words are formed from pulses of various widths, the speed and direction of a moving target as well as diagnostic data can be communicated to various circuits which can utilize this information. In one embodiment, when the target is moving in a first direction, the protocol provides speed and direction information on every edge or pole boundary of the target and the diagnostic data is decoded by measuring the width of each pulse. The diagnostic data can include, but is not limited to, status of an air gap between a sensor and a target and a temperature of the environment in which the sensor or target is disposed. In one embodiment, the first logic data bit corresponds to a logic zero data bit and the second logic data bit corresponds to a logic one data bit and the data string signal protocol includes a start data sequence (SDS) and a plurality of data words, each of the data words separated by a data word separator.
In accordance with a further aspect of the present invention, a magnetic article proximity detector includes a magnetic field sensor for providing an output signal proportional to a magnetic field and a detection circuit to detect at least one of: (a) a parameter of the environment in which the magnetic article is disposed; (b) a parameter of the environment in which the magnetic article proximity detector is disposed; (c) a parameter related to a relationship between the magnetic article proximity detector and the magnetic article; and (d) a parameter of the magnetic article. The magnetic article proximity detector further includes an output control circuit coupled to receive one or more signals from the detection circuit. In response to the one or more signals, the detection circuit provides a data string in accordance with a predetermined protocol in which a first logic data bit having a first logic value in the data string is provided from a single pulse having a first pulse characteristic and a second logic data bit having a second logic value in the data string, is provided from a single pulse having a second pulse characteristic with the second pulse width being related to the first pulse characteristic.
With this particular arrangement, a magnetic article proximity detector is provided which utilizes a protocol which conveys information in the form of data words. In one embodiment, each of the data words include two logic data bits for a total of four possible states for each word. In other embodiments, more than two bits per word can be used in which case more than four states for each data word will exist. In one automotive application, the detector detects information about a rotating ferrous target and provides the information to an engine control unit. Also, in one embodiment the pulse characteristic corresponds to one of a pulse width, a current level and a voltage level. The detection circuit can include at least one of a speed detection circuit, a direction detection circuit, a temperature detection circuit, an air gap detection circuit and a voltage detection circuit. In response to signals fed thereto from the magnetic field sensor, one or more of the speed, direction, temperature, air gap and voltage detection circuits provides an output pulse stream providing speed, direction, and diagnostic information in accordance with a predetermined protocol which includes a data string having a plurality of pulses, one pulse for each edge of a detected target wherein a first logic data bit is represented by a pulse having a first pulse width and a second logic data bit is represented by a pulse having a second pulse width wherein a first one of the first and second pulse widths is a multiple of a second one of the first and second pulse widths.
In accordance with a still further aspect of the present invention, a method includes providing a start data sequence and providing a first data word having a plurality of pulses with each of the plurality of pulses having a pulse characteristic with a first pulse characteristic value corresponding to a first logic value and a second pulse characteristic value corresponding to a second logic value. In one embodiment, the pulse characteristic corresponds to one of a pulse width, a current level and a voltage level and each pulse corresponds to a data bit having a logic value. In one embodiment, the first data word is a first word of a plurality of words in a data string and each data bit is provided from a single pulse having a first or second logic value. With this particular arrangement, a method for conveying information via data words is provided.